Central respiratory chemoreception is the process by which CNS pCO2 activates respiration. This process is essential to maintain breathing automaticity during sleep. Its dysregulation probably contributes to common forms of sleep-disordered breathing (e.g. obstructive sleep apnea, Cheyne-Stokes breathing) and is the most probable cause of the central congenital hypoventilation syndrome (CCHS), a rare genetic disease due to a mutation of the homeobox transcription factor Phox2b. The cellular and molecular underpinning of central respiratory chemoreception is still poorly understood although some of the most important neurons are assumed to reside somewhere close to the ventral surface of the medulla oblongata (VMS). In the past three years, we have identified within a region of the VMS called retrotrapezoid nucleus (RTN) a cluster of glutamatergic neurons that have properties consistent with central chemoreceptors. The potential importance of these neurons was highlighted by our recent finding that they express Phox2b, the transcription factor that is mutated in CCHS. The present project is designed to explore the role of these Phox2b-expressing neurons in breathing in general and in respiratory chemoreception in particular. The research is organized around three Aims. In Aim 1 we seek further evidence that these RTN neurons regulate the respiratory rhythm and pattern generator. More specifically, we propose to determine whether selective lesions of the Phox2b-expressing neurons produce the expected reduction in central chemoreflexes and we also seek to identify which respiratory neurons are synaptic targets of these cells. In Aim 2 we ask whether these Phox2b- expressing neurons are activated by acidification in vitro, a property required for such cells to qualify as central chemoreceptors. Finally, in Aim 3 we propose to study some of the brain inputs that regulate the activity and pH-sensitivity of the Phox2b-expressing neurons of the RTN with a focus on lung mechanoreceptors and on inputs from the serotonergic system. In short, this research has two objectives: first to increase current understanding of the cellular mechanisms responsible for central respiratory chemoreception and second, to help understand the causes of CCHS.